Researchers improve tabletop EUV laser system

BOULDER, Colo.  Researchers at the University of Colorado have boosted the output power and reduced the beam wavelength of their tabletop extreme ultraviolet (EUV) laser system built with off-the-shelf components without altering its $5,000 cost, opening the system to a new range of possible applications, including nanoscale chip lithography and microscopy.

By modifying their previous tabletop EUV system, professors Margaret Murnane and Henry Kapteyn increased the system's output power from 100 watts to 1 megawatt, and downsized the wavelength of its laser beam from 30 nanometers to 7 nm, making it applicable to next-generation 13-nm EUV lithography.

"Previously we showed that we could generate EUV beams from a hollow gas-filled fiber that were laser-like at 30 nm. Now we have put modulations on the fiber that allowed us to generate these laser-like beams down to much shorter wavelengths  as small as 7 nm," Murnane said. "These shorter wavelengths are of interest for technological and biological and materials applications."

Murnane and Kapteyn pioneered the high-harmonic generation (HHG) method of boosting the frequency of a laser to the nanoscale EUV range. HHG fires femtosecond pulses of visible light laser into a hollow waveguide filled with a gas, ionizing it, which then transfers energy from the fundamental frequency into very high harmonic orders.

The researchers added internal "humps" to their previous system that act to modulate the waves into reinforcing one another, thus focusing the system's beam to 7 nm and increasing its peak output to over a megawatt.

"By creating ripples in the diameter of the waveguide, we coaxed the light waves from the laser into traveling at the same speed as EUV beams  a result called phase-matching," Murnane said.

In the future, the researchers hope to downsize the system's wavelength again, this time to 4 nm  what they call the "water-window"  a region of the spectrum where imaging living biological structures would be possible.

Their research was supported by the National Science Foundation along with the U.S. Department of Energy. An earlier NSF grant enabled them to develop their EUV beam's femtosecond laser source  a simpler, yet shorter-pulse version of a $100,000 commercial laser that can be built with only $5,000 in parts.